The present invention relates to vacuum and gas-filled electrothermal units and, more specifically, to continuous electric furnace for chemical heat treatment of steel parts.
The invention can most advantageously be used for vacuum carburizing in electrical engineering, automotive industry, and in other fields of machine building. In addition, the invention can be employed in chemical heat treatment of products manufactured by powder metallurgy techniques.
The invention is also applicable in such processes as vacuum brazing and impregnation of surfaces with various elements, for example, silicon, titanium, boron, etc.
The maximum efficiency in chemical heat treatment of parts is achieved through the use of continuous furnaces.
Known in the art is a case-hardening pusher-type furnace (cf. the handbook "Electrothermic Equipment" edited by A. P. Altgauzen, Moscow, 1967, p. 178, FIG. 5-133), comprising charging and discharging mechanisms, a case-hardening chamber, and a cooling chamber arranged in line. The furnace incorporates a pusher serving to supply parts to the case-hardening chamber, and a moving mechanism serving to feed the parts into the cooling chamber. The heating chamber is furnished with a fan for circulating the medium during the carburizing procedure. Such furnaces are employed by the Aichelin and Halcroft companies.
The furnace operates as follows. The parts are loaded into containers. A container filled with the parts is placed on the charging mechanism, and the pusher moves it into the casehardening chamber. A chamber door opens, and the parts are fed through the door opening to the heating chamber wherein they are heated to the carburizing temperature while travelling through the chamber. The furnace commonly employs a flame curtain to maintain the medium in the case-hardening chamber. The case-hardening chamber is filled with saturating gas supplied thereto. On completion of the carburizing procedure, discharge doors open, and the charge is forwarded into the cooling chamber.
The furnace is a fire- and explosion-hazardous unit which contaminates the workshop environment, and special-function ventilation is required. The equipment set of the furnace must include environment-control generators with a high degree of purification, and must also incorporate natural gas supply appliances of the furnace. The carbon potential must be accurately controlled inside the furnace, since it can be easily disturbed during charging and discharging operations. Besides, the structural members of the furnace (including heaters and lining) are not adapted for increasing the temperature in the furnace because of heavy soot precipitation leading to failures of the heaters, hence, the furnace output drops.
There is known in the art a continuous vacuum furnace for heat treatment with provisions for vacuum cooling and further inert gas cooling (cf. Data Sheet No. 771, type LV, model CVCQ-202436, Hays company), comprising a charging and a discharging chambers equipped with vacuum seals, a case-hardening chamber with heat insulation made of fiber material, and a vacuum cooling chamber.
The furnace operates as follows. A container filled with products is charged into the charging chamber, with the seal of the case-hardening chamber closed. Then the charging chamber door is shut, the chamber is evacuated for a predetermined vacuum, and a vacuum seal wherethrough the charging chamber communicates with the case-hardening chamber opens for passing the container to the case-hardening chamber wherein the products in the charge are heated to the carburizing temperature during movement of the container through the chamber, and are impregnated with carbon. On completion of the full case-hardening cycle, the products are forwarded to the cooling area wherefrom they are moved through another vacuum seal to the inert gas quick-cooling area, and then to the discharging chamber.
The construction of the prior-art furnace does not provide for producing a uniform carburized layer because the products located in the container during heating of the charge do not absorb carbon equally at different temperatures, hence, the carburized layer thickness varies. Besides, the furnace of the foregoing design requires large amounts of carburizing gas and electric power since the furnace must be filled with reactive gas to capacity.
Also known in the art is a pusher-type furnace (cf. USSR Inventor's Certificate No. 601317, C21D 9/00, 1978), comprising a charging chamber, a discharging chamber, a heating chamber, an intermediate chamber equipped with heating elements and attached by a holder to a pusher serving to move the intermediate chamber in a vertical direction, and a cooling chamber.
The furnace operates as follows. Products to be treated are placed into pans. Each pan is installed into the charging chamber, and the pusher moves the pan into the heating chamber after the seal opens, wherein the pan is held during a predetermined time. Then the next pan is charged into the furnace. During each push, the pan heats up gradually and comes under the intermediate chamber. When the intermediate chamber moves downward, a platform of the pan appears to be in the heat treatment area. After a certain exposive period, the pusher returns the intermediate chamber to the initial position, the pan with the products remains in place, for a certain period of time, and then the pusher sends the pan to the cooling chamber. From the cooling chamber the pan is supplied to the discharging chamber. After that, the procedure is repeated. The intermediate chamber of the above design is not adapted for easy sealing of the processing chamber required for chemical heat treatment accomplished in the vacuum furnace. Besides, the construction of the furnace is complicated because of the above-mentioned design of the movable chamber.
There is also known a continuous electric furnace for chemical heat treatment of parts (cf. USSR Inventor's Certificate No. 456569, F 27 B 9/22, 1976), comprising a parts heating chamber separated into different heating areas, a separate heat-insulated high-temperature processing chamber connected to a parts vertical supply chamber, and a cooling chamber. The heating chamber, the chamber of vertical supply of parts to the processing chamber, and the cooling chamber are aligned and the processing chamber is located outside of the line for continuous horizontal supply of parts.
The furnace operates as follows. Containers with parts fed to the heating chamber travel along the chamber and heat up gradually. After the parts are heated to a predetermined temperature, they are fed to the vertical supply chamber wherefrom the parts are moved to the processing chamber serving for treatment of parts at a preset temperature. On completion of treatment inside the processing chamber, the containers with parts move down and are sent further to the cooling chamber.
There are no provisions in the prior-art furnace for sealing the processing chamber required to permit efficient chemical heat treatment of parts.